All railroads operating in this state shall erect stands at a point not less than one hundred and fifty feet from either approach of their bridges.  From these stands light ropes shall be hung across the entire width of their track and properly knotted and hanging so low as to touch the head or body of any train-hand in ample time to notify him of his near approach to the bridge, so as to enable him to take the necessary precautionary means to avoid the possibility of injury.